In semiconductor integrated circuits, the formation of metal interconnect layers is important to the proper operation of such devices. These metal interconnect signal lines connect to lower conductive layers and "active" device regions of the integrated circuit through vias or through contact windows. Metal interconnect lines also serve as lines or runners on surface layers of integrated circuits to connect to other device areas. For best operation of the device, the metal must have sufficient conductivity to carry the electric signal and at the same time possess the ability to adhere to adjacent layers.
As the semiconductor industry attempts to reduce line widths to create smaller, faster devices, new materials will be used to overcome many of the physical limitations required by these reduced line widths. To overcome the interconnect resistance and improve electromigration resistance, many semiconductor manufacturers are turning to copper for the metal layers. However, in the past the use of copper in semiconductor devices has been limited. Copper atoms will readily diffuse through silicon causing contamination problems that cause leakage currents at p-n junctions, failure of dielectric layers, and deterioration of carrier lifetime. Therefore, unsuccessful containment of copper can have fatal effects on a semiconductor device. Copper is also subject to reaction with atmospheric oxygen or moisture during the formation of metal layers and interconnect lines. Such adverse reactions form undesirable compounds having lower conductivity and poor adhesion to other materials used in semiconductor fabrication. Accordingly, copper processing technology is an extremely important and new problem for the semiconductor industry.
To form suitable interconnects, copper metal surface layers formed on the semiconductor device must be free of any oxidized regions. Once the copper metal layer is formed and subject to an oxidizing environment, such as air, the resulting oxidized areas must be reduced back to unoxidized copper or subsequent layers will not adhere. Therefore, the subsequent process must first have an oxide reduction step where the copper layers and interconnects are subjected to a copper reduction reaction to convert any oxidized portion back to copper metal. Such reduction processes must be carefully controlled. Incomplete reduction results in a metal surface containing residual oxidized portions. Yet, if the reaction is allowed to proceed too long, the surface of the metal layers or interconnects become pitted. In either case, the performance of the device is adversely affected. Therefore, conditions that result in removal of the oxide regions without related pitting must be determined.
To solve the problems associated with reduction of oxidized copper on layers and interconnects, several approaches can be used. Currently, conditions for producing suitable copper metal surfaces and interconnects are determined manually. After a time the device is inspected to determine if the reduction is complete. The process is repeated until all oxide is removed. However, determining the endpoint of the reduction reaction in this way is both time consuming and expensive.
Accordingly, what is needed in the art is an automated method for detecting the endpoint of a reduction reaction in the fabrication of a semiconductor device. The method of the present invention addresses these needs.